25,695 research outputs found
Higher-order Lorentz-invariance violation, quantum gravity and fine-tuning
The issue of Lorentz fine-tuning in effective theories containing
higher-order operators is studied. To this end, we focus on the Myers-Pospelov
extension of QED with dimension-five operators in the photon sector and
standard fermions. We compute the fermion self-energy at one-loop order
considering its even and odd contributions. In the even sector we find
small radiative corrections to the usual parameters of QED which also turn to
be finite. In the odd sector the axial operator is shown to contain
unsuppressed effects of Lorentz violation leading to a possible fine-tuning. We
use dimensional regularization to deal with the divergencies and a generic
preferred four-vector. Taking the first steps in the renormalization procedure
for Lorentz violating theories we arrive to acceptable small corrections
allowing to set the bound .Comment: 11 pages, new version with the correct pole extractio
Unitarity, ghosts and nonminimal terms in Lorentz violating QED
The unitarity of a Lorentz-invariance violating QED model with higher-order
Myers and Pospelov photons coupled to standard fermions is studied. As
expected, we find ghost states associated to the higher-order terms that may
lead to the loss of unitarity. An explicit calculation to check perturbative
unitarity in the process of electron-positron scattering is performed and it is
found to be possible to be preserved.Comment: Presented at the Sixth Meeting on CPT and Lorentz Symmetry,
Bloomington, Indiana, June 17-21, 201
Testing symmetries in effective models of higher derivative field theories
Higher derivative field theories with interactions raise serious doubts about
their validity due to severe energy instabilities. In many cases the
implementation of a direct perturbation treatment to excise the dangerous
negative-energies from a higher derivative field theory may lead to violations
of Lorentz and other symmetries. In this work we study a perturbative
formulation for higher derivative field theories that allows the construction
of a low-energy effective field theory being a genuine perturbations over the
ordinary-derivative theory and having a positive-defined Hamiltonian. We show
that some discrete symmetries are recovered in the low-energy effective theory
when the perturbative method to reduce the negative-energy degrees of freedom
from the higher derivative theory is applied. In particular, we focus on the
higher derivative Maxwell-Chern-Simons model which is a Lorentz invariant and
parity-odd theory in 2+1 dimensions. The parity violation arises in the
effective action of QED as a quantum correction from the massive fermionic
sector. We obtain the effective field theory which remains Lorentz invariant,
but parity invariant to the order considered in the perturbative expansion.Comment: 13 pages, Sec. III, additional references added, P symmetry revised,
accepted for publication in PR
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